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The present invention relates to operations involving the drilling of wells in an offshore environment. In particular, the present invention relates to improved methods and an apparatus for installing conductor casing for offshore wells. More specifically, the present invention relates to method and apparatus for driving conductor casing into the seafloor using a retrievable suction embedment chamber assembly.
Hydrocarbon reservoirs are found in special formations of rock usually located far underground. These reservoirs are exploited by drilling wells into the ground to facilitate the extraction of the hydrocarbons. As wells are drilled, many different formations of rock are encountered. Some of these formations are very hard, while others are loose or sandy. Shallow formations often contain water. In order to protect the integrity of the well as it is drilled, lengths of pipe, known as casing, are placed, or set, into the well. The casing acts as a barrier to prevent the sides of the well from caving in, to prevent the movement of fluid, like water or hydrocarbons, from one formation to another, and to increase efficiency of the well if it used to produce hydrocarbons.
A length of a casing is known as a string. A typical well will contain several strings of casing, each with a different diameter. The largest diameter casing will be at the top of the well with each successive casing string having a smaller diameter so that it can be moved through the casing already in the well. Conductor casing, also known as conductor pipe, is the largest diameter casing and therefore the first tubular placed when drilling a well. The purpose of the conductor casing is to prevent the loose, shallow formations from falling into the wellbore.
The diameter of the casing strings is determined by several factors including depth of the well and the type of formation being drilled in. As each string of casing is set, cement is pumped into the hole and around the outside of the casing to lock the casing in place and seal off the surrounding formation. When the well reaches the formations containing hydrocarbons, it is in effect a continuous, sealed conduit to the surface. Therefore, any hydrocarbons produced can not migrate into other formations or into ground water. Large valves, or pressure control equipment such as a blowout preventer or a production tree, are attached to the casing at the top of the well, known as the wellhead, to control the flow of material out of the well.
The formations of rock that contain hydrocarbons are found all over the world. Many of the recent efforts to find and produce these hydrocarbons have focused on formations located under water. In subsea wells, the conductor casing extends through an initial layer of mud and silt and provides a solid foundation for the well. The conductor casing is usually large diameter pipe ranging from 6 to 60 inches in diameter and can be several hundred feet long.
Early offshore wells were drilled in relatively shallow water a only few hundred feet deep. For wells drilled at these shallow depths, the conductor casing typically extends to the surface of the water and is attached to a platform. Conductor casing for these shallow water wells can be driven using equipment at the surface. For wells in deeper water, the conductor casing terminates at or near the seafloor. Because the conductor casing does not extend to the surface of the water, these deep-water wells require that the casing driving mechanisms be below the surface and typically at the seafloor. There are wells being drilled today in water depths up to two miles.
Prior art methods for installing conductor casing offshore include driving or hammering the conductor casing into the seafloor with a pile driver, rotary drilling with a drill bit to create a hole in the seafloor for the casing, and using high pressure liquids to wash, or jet, out a cavity in the seafloor for the casing. Prior art methods of installation require the heavy hoisting, rotary, or pumping equipment that can only be provided by the drilling vessel. The present invention overcomes these and other drawbacks of the prior art.
The embodiments of the present invention provide methods and apparatus for setting conductor casing for subsea wells using hydrostatic pressure as the driving force. The present invention allows conductor casing to be set more efficiently, and from smaller vessels, than the prior art methods. In one embodiment, a string of conductor casing is attached to a large chamber that is open at the base and substantially closed at the top. The conductor casing is placed through an opening in the top of the chamber and is attached to the chamber by an attachment mechanism which may be a ratcheting mechanism. The chamber is fitted with a device, such as a pump, for evacuating or filling the chamber from the top.
This assembly is lowered to the seafloor using a crane or winch mounted on a vessel. Once on the seafloor, the chamber is allowed to settle into the seafloor under its own weight, where the mud and silt of the seafloor create a seal around the base of the chamber. Once the assembly has settled into the seafloor, the pump is activated, and material, including water and mud, is pumped out of the chamber. As the pressure inside the chamber drops, the hydrostatic pressure operating on the outside of the chamber forces the chamber and the attached conductor casing into the seafloor.
When the chamber has moved as far into the seafloor as desired, the attachment mechanism can be released and water pumped into the chamber, creating a differential pressure that pushes the chamber out of the seafloor and toward the surface. Because the conductor casing is released from the chamber, the surface friction between the casing and the mud will hold the casing in place. In one embodiment, the chamber is moved up the casing and the attachment mechanism is re-engaged. The pump is reversed, lowering the pressure inside the chamber, and the chamber is again forced into the seafloor, pulling the casing with it. This sequence can be repeated until the casing reaches the desired depth. Once the casing reaches the desired depth, the attachment mechanism can be disengaged and the chamber moved free of the seafloor, where it can be retrieved to the surface or used to place additional strings of conductor casing.
The only equipment required to operate the above described embodiment is a lifting device, such as a crane or a winch, and a power supply to the pump and attachment mechanism. This equipment is typically available on many vessels that service the offshore petroleum industry. This allows flexibility in choosing the most economical vessel to use to set conductor casing.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings.